The embodiments described herein relate generally to an electric machine, and more specifically, to a bearing assembly with a relative motion [bearing creep] impeding device associated with the electric machine.
An electric machine is typically in the form of an electric generator or an electric motor. The machine typically has a centrally located shaft that rotates relative to the machine. Electrical energy applied to coils within the motor initiates this relative motion which transfers the power to the shaft and, alternatively, mechanical energy from the relative motion of the generator excites electrical energy into the coils. For expediency the machine will be described hereinafter as a motor. It should be appreciated that a motor may operate as a generator and vice versa.
A stationary assembly, also referred to as a stator, includes a stator core and coils or windings positioned around portions of the stator core. It is these coils to which energy is applied to initiate this relative motion which transfers the power to the shaft. These coils are formed by winding wire, typically copper, aluminum or a combination thereof, about a central core to form the winding or coil.
The centrally located shaft supports a rotor. The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator. Motor efficiency is improved by decreasing the degree of slip between the rotor and the stator for a given load. One way to decrease the slip is by increasing the mass of the rotor. The rotor includes conductors, conductor bars and end-plates which transfer current, magnetic field and torque to the rotor and consequently torque to the shaft.
The shaft rotatably supports the substantial weight of the rotor within the electric machine. At least one bearing and typically a pair of spaced apart bearings support the shaft within a housing of the electric machine. The bearings may be any bearing capable of supporting the loads involved and of enduring the rotational speeds of the motor. Typically modern electric machines utilize rolling element bearings, typically ball bearings to support the shaft and rotor. The ball bearings include an inner ring and an outer ring separated by a series of spherical elements or balls. While the shaft is typically rigidly secured to the inner ring, the outer ring is permitted to move axial in the housing, to accommodate various phenomenon including temperature changes, dissimilar materials, and tolerance stacks.
When the bearing is positioned with the shaft horizontal to the horizon or ground, a substantial radial load from the rotor is applied to the bearing. This radial load serves to inhibit relative motion or rotation of the bearing outer ring in the housing. However when the bearing is positioned with the shaft vertical with respect to the horizon or ground, radial loads can be zero or minimal, permitting relative slippage to occur between the bearing outer ring and the housing.
The initial relative slippage is exasperated by the formation of metal debris that forms a lapping compound between the bearing outer ring and the housing bore. This lapping may quickly greatly enlarge the housing bore, causing excessive noise and excessive movement between the motor stator and the motor rotor, resulting in rotor/stator strike. The noise and strike may lead to early bearing and resultant motor failure.
Many practical applications utilize motors with vertical shafts. For example, pumps for pools and spas and cooling fans, particularly those to cool air conditioning compressors. These applications are plagued with bearing failures caused by the relative motion of the motor bearing outer ring in the motor housing, also known as bearing creep. Typically, vertical cooling fan motors have a vertical shaft extending upwardly from the motor and from which a fan is attached. The motor typically has an upper unconstrained bearing and a lower constrained bearing. The upper unconstrained bearing, being closer to the fan, has a greater tendency for creep, but creep can occur in any bearing application, particularly those where accommodation is made for the bearing to move axially with respect to its housing.
Various methods are used to limit the bearing creep. One method is to eliminate any relative motion, including axial motion, by providing an interference fit between the bearing outer ring and the housing or by using an adhesive between the outer ring and housing. However any solution that eliminates all relative motion has the disadvantage of not accommodating various phenomenon including temperature changes, dissimilar materials, and tolerance stacks that affect the relative axial position of the inner ring with respect to the outer ring. Such a solution may result in excessive bearing preloads and reduced bearing life. Further, interference fit between both bearing races is not recommended by bearing manufacturers. Other methods, such as placing an o-ring, a polymer ring, an EC (expansion compensating bearing), between the bearing and the housing are either ineffective or not sufficiently durable. The present invention is directed to alleviate at least some of these problems with the prior art.